Light affects various aspects of growth and development in higher plants throughout their life cycles, from germination to flowering (Fankhauser and Chory, 1997). Seedlings grown in the dark undergo skotomorphogenesis, characterized by elongated hypocotyls, yellow and closed cotyledons. In response to light, seedlings undergo photomorphogenesis; hypocotyls cease elongating, cotyledons become green and unfolded, and the seedlings become photosynthesis competent.
A number of photoreceptors controlling light-dependent development, including red (R) and far-red (FR) light absorbing phytochromes, blue light receptors, cryptochromes, and phototropins have been characterized (Furuya, 1993; Lin, 2000). Among these, the phytochromes are the best characterized. Phytochromes exist as two photo-interconvertible forms, Pr (R light-absorbing phytochrome) and Pfr (FR light-absorbing phytochrome), depending on light conditions (Butler et al., 1959). In higher plants, phytochrome apoproteins are encoded by a small gene family, such as PHYA-E in Arabidopsis (Sharrock and Quail, 1989). Mutational and transgenic approaches have revealed that individual phytochromes have overlapping but distinct functions (Reed et al., 1994; Quail et al., 1995; Furuya and Schäfer, 1996; Whitelam and Devlin, 1997). In particular, phyA is a primary photoreceptor for FR-high irradiance response (HIR) and very low fluence response (VLFR), whereas phyB is a primary photoreceptor for R-HIR and R-low fluence response (LFR).
The downstream components of phytochrome signaling have been extensively characterized. Light-dependent post-translational modifications and subcellular localization of phytochromes have also been implicated to play a role in phytochrome downstream signaling (Lapko et al., 1997; Yeh and Lagarias, 1998; Kircher et al., 1999; Yamaguchi et al., 1999; Kim et al., 2002b). Several phytochrome-interacting molecules have been identified; implying that phytochrome may utilize multiple interacting partners to induced various photoresponses (Quail, 2002b).
Mutant screening using light-dependent seedling development has been fruitful to reveal a number of phytochrome-signaling components, including photoreceptors (Neff et al., 2000). One class of mutants includes the ones that exhibit altered photo-responses under different light conditions, defining light-dependent positive and negative regulators. Several of these are transcription factors, including two basic helix-loop-helix (bHLH) proteins, HFR1 and PIF4 (Fairchild et al., 2000; Fankhauser and Chory, 2000; Soh et al., 2000; Huq and Quail, 2002), a bZIP protein, HY5 (Oyama et al., 1997), and a MYB protein, LAF1 (Ballesteros et al., 2001), that have been shown to regulate not only distinct but overlapping subsets of photoresponses. EID1 and SPA1, phyA-dependent negative regulators, have been implicated to control protein stability in the nucleus (Hoecker et al., 1999; Dieterle et al., 2001; Hoecker and Quail, 2001). The other class of mutants revealed a group of repressors of photomorphogenesis, COP/DET/FUS. The cop/det/fus mutations confer photomorphogenic development even in the absence of light, including shortened hypocotyls, expanded cotyledons, and increased expression of light-inducible genes (Chory et al., 1989; Wei and Deng, 1996). Recent studies proposed that DET1, a nuclear protein, regulates gene expression via chromatin remodeling, which could control the accessibility of a promoter to specific transcription factors, for example (Benvenuto et al., 2002; Schroeder et al., 2002). COP1 encodes a RING-finger protein with WD 40 repeats whose nuclear localization is negatively regulated by light (Deng et al., 1992; von Arnim and Deng, 1994). In darkness, COP1 interacts with, and down-regulates several transcription factors that act as positive components in light signaling (Ang et al., 1998; Hardtke et al., 2000; Osterlund et al., 2000; Yamamoto et al., 1998, 2000). Other cop/det/fus loci encode an ubiquitin-conjugating enzyme or components of the COP9 signalosome complex, which was proposed to function in the proteasome-mediated protein degradation (Suzuki et al., 2002; Serino et al., 2003). Together, these findings led to the hypothesis that the primary mode of phytochrome signaling for seedling development involves post-translational regulation on the nuclear transcription (Nagy and Schäfer, 2002). Despite extensive list of phytochrome signaling components, the molecular mechanisms by which these components mediate phytochrome downstream signaling mechanism are still poorly understood (Nagy and Schäfer, 2002). In particular, it is notable that no molecular components have been identified to mediate phytochrome-dependent germination.
Previously, HFR1, a bHLH protein was shown to be required for a subset of phyA-dependent responses and act downstream of COP1 (Fairchild et al., 2000; Fankhauser and Chory, 2000; Soh et al., 2000; Kim et al., 2002b). Here we used a transgenic approach to further investigate the role of HFR1 in light signaling.